1) Field of the Invention
The present invention relates to a thermal transfer dye image-receiving sheet. More particularly, the present invention relates to a thermal transfer dye image-receiving sheet capable of receiving and fixing thereon thermally transferred dye or ink images or pictures in a clear and sharp form without a thermal curling thereof, capable of recording thereon continuous tone full colored images or pictures at a high resolution and a high tone reproductivity, and optionally capable of preventing stains of the back surface of the dye image-receiving sheet, caused by a dye or ink.
2) Description of the Related Arts
It is known that new types of color printers, for example, relatively compact thermal printing systems having a thermal head, enable a printing of clear colored images or pictures by a thermal transfer of the colored images or pictures of a thermomelting ink or sublimating dye onto an image-receiving sheet, and there is great interest in the further development and utilization of these printing systems, especially the sublimating dye colored image or picture-thermal transfer printing systems.
In the operation of the sublimating dye image or picture-thermal transfer printing system, an image-receiving sheet having a polyester resin layer, on which the sublimated dye is easily dyed, is superimposed on an ink sheet comprising a support sheet consisting of a thin plastic sheet and a sublimating dye ink layer formed on a surface of the support sheet, in a manner such that the surface of the polyester resin layer of the image-receiving sheet comes into contact with the surface of the ink layer of the ink sheet, and the ink sheet is locally heated imagewise by a thermal head in accordance with electric signals corresponding to the images or pictures to be printed, to thermally transfer the ink images or pictures composed of the sublimated dye, and having a color density corresponding to the amount of heat applied to the ink sheet on the polyester resin layer of the image-receiving sheet.
It is also known that a support sheet comprising a sheet substrate and a coating layer formed by bonding a bi-axially oriented plastic film consisting of a mixture of an inorganic pigment and a polyolefin resin and having a multilayered structure to the sheet substrate surface enables thermal transfer image-receiving sheets to receive thermally transferred images or pictures having a high quality from a printing system having a thermal head.
In the image-receiving sheet for the sublimating dye thermal transfer printing system, the above-mentioned support sheet is coated with a thermal transfer image-receiving layer comprising, as a principal component, a polyester resin.
The record sheet or image-receiving sheet having the above-mentioned support sheet has an even thickness, a high softness, and a lower thermal conductivity than that of paper composed of cellulose fibers, and therefore, is advantageous in that images or pictures having a high uniformity and color density can be formed thereon. Nevertheless, where the coating layer in the support sheet is formed from a bi-axially oriented plastic film comprising, as a principal component, a polyolefin, for example, polypropylene resin, and having a multilayered structure, and ink or dye images or pictures are thermally transferred by heat from a thermal head to the polyester resin coating layer in the image-receiving sheet, the multilayer structured polyolefin resin coating film in the support sheet is heated by the thermal head so that a drawing stress held in the polyolefin resin coating film is released, and thus the polypropylene resin coating film layer shrinks. This shrinkage of the polyolefin resin coating layer causes the image-receiving sheet to be curled and a number of wrinkles to be formed thereon, so that the forwarding of the sheet in the printing system is hindered by the curls or wrinkles on the sheet and the resultant prints have a reduced commercial value.
To eliminate the above-mentioned disadvantages, a new type of support sheet was provided by coating two surfaces of a sheet substrate consisting of, for example, a paper sheet, and having a relatively small heat shrinkage with the multilayer-structured plastic coating films. This type of the support sheet effectively prevents the formation of wrinkles on the image-receiving sheet due to the heat shrinkage of the plastic coating films, but since two coating films having different heat shrinkages are laminated on a sheet substrate, and the thermal transfer operation is applied to one side surface of the image-receiving sheet, the image-receiving sheet is locally shrunk, and thus is naturally not free from curl-formation. Especially, in the sublimating dye thermal transfer printing system, a large quantity of heat is applied to the image-receiving sheet, and therefore, the abovementioned problems often occur on the image-receiving sheet.
The sublimating dye thermal transfer printing system is a mainstream printing system among small size non-impact full colored image-printing systems, and thus is often used as a printer for small size electronic cameras or video printers. Therefore, there is an urgent demand for the provision of a new type of thermal transfer image-receiving sheet which can form clear images or pictures thereon without a thermal deformation thereof, even when used for the sublimating dye thermal transfer printing system in which a large quantity of heat is applied to the image-receiving sheet.
When a paper sheet comprising a cellulose pulp is used as a substrate sheet, the resultant image-receiving sheet is disadvantageous in that the images or pictures formed thereon have fiber-shaped marks or patches due to the use of the substrate paper sheet or due to the uneven adhesion of the coating layers with the substrate paper sheet, and thus the reproductivity of images is lowered.
To eliminate the above-mentioned disadvantages, an attempt was made to lower the thermal shrinkage of the multilayer-structured plastic resin film by heat-treating. That is, the multilayer-structured film was continuously brought into contact with a heating roller or passed through a heating oven, whereby the residual drawing stress on the film is released and the thermal shrinkage of the film was lowered. Nevertheless, when the long film was continuously heated while moving the film in the longitudinal direction thereof, it was found that the film shrunk in the transversal direction thereof and wrinkles and slacks were created on the film. Also, the multilayer-structured film had a low thermal conductivity, and a long time was required for completing the heat treatment. Therefore, it is difficult to effectively and evenly carried out the heat treatment for a multilayer structured film, with a high reproductivity, and the resultant heat treated film often has an uneven rough surface thereof.
In another attempt, a drawn or undrawn film comprising a thermoplastic resin and having a low thermal shrinkage, for example, polyester, polyolefin or polyamide, was employed as a substrate sheet for a dye image-receiving sheet. Especially, an attempt was made to use, as a substrate sheet, a film comprising a polyethylene terephthalate resin which may be modified with a modifying agent or copolymerized with a comonomer, and having a high resistance to deformation, for example, stretching and bending, and a uniform thickness.
When the polyethylene terephthalate resin film per se is employed as a dye image-receiving sheet, this sheet is advantageous in that substantially no curl is generated on the sheet during the thermal transfer printing operation, and the resultant transferred images or pictures have a uniform shading and quality. Therefore, it is considered that, because an oriented film consisting of a mixture of a polyethylene terephthalate resin with a white filler (pigment) has a high whiteness and opacity, it is preferable as a dye image-receiving sheet capable of receiving the thermal transferred images in a clear and sharp form.
Nevertheless, it was found that the polyester resin film is disadvantageous in that it is costly, exhibits a poor sensitivity when receiving the transferred images, and accordingly, the received images have a low color density due to the high thermal conductivity thereof. Further, it has a poor reliability with regard to the smooth movement thereof in the printer, due to a high modulus of elasticity and a high resistance to deformation (bending) thereof, and therefore, the transferred images are sometimes display an uneven color density or shading and are not clearly defined.
In still another attempt, a new type of dye image-receiving layer was developed. For example, Japanese Unexamined Patent Publication (Kokai) No. 62-244696 discloses a phenyl-modified polyester resin, and Japanese Unexamined Patent Publication (Kokai) No. 63-7971 discloses a polyester resin modified with a phenyl radical-containing alcohol compound. These new types of modified polyester resins are soluble in an organic solvent and useful for forming a dye image-receiving layer having a superior capability of receiving thereon a large amount of clearly defined dye images at a high transfer speed, and having an enhanced storage durability or stability.
Nevertheless, the resultant dye image-receiving sheet having an organic solvent-soluble polymeric layer for receiving dye images and at least one thermoplastic resin layer exhibits a high electrification property, and thus is disadvantageous in that the dye-image receiving sheet has a poor reliability with regard to a smooth feeding, movement, and delivery thereof in a printer. Also when a plurality of the image-transferred sheets are superposed one on the other, and stored in this state, an undesirable electric charge is generated on the sheets due to friction therebetween. Therefore, the printed sheets are electrically adhered (blocked) to each other and scratched by the friction therebetween, and thus the commercial value thereof is reduced.
To prevent the above-mentioned disadvantages due to electrification, usually an anti-static agent is applied to at least one face of the dye image-receiving sheet or to at least one face of a dye sheet. Nevertheless, in the thermal transfer printer, a plurality of dye image-receiving sheets are fed one by one to the printing operation, and thus it is difficult to completely prevent the above sheet-feeding problem, caused by the electrification of the dye image-receiving sheet, by only the application of the anti-static agent. Also, the printed sheets are stored and employed over a long period of time, and therefore, to prevent an adhesion of dust to the printed sheets, the effect of the anti-static agent applied to the sheets must be maintained for a long time. Also, even where the anti-static treatment is applied to the dye image-receiving sheets, when the sheets are stored and employed under a high humidity condition, the anti-static effect is not effectively generated on the sheets, and thus the above sheet feeding problem often occurs.
Accordingly, there is a demand for the provision of a new type of thermal transfer dye image-receiving sheet which is resistant to an electrification thereof while stored and employed.
In the printing operation, a number of dye image-receiving sheets is stored in the superimposed form, one on the other, in the printer and fed one by one to a printing step. Therefore, the image-receiving surfaces of the sheets are sometimes scratched by the back surfaces of adjacent sheets, whereby the commercial value of the resultant prints is significantly lowered.
Accordingly, there is a demand for the provision of a dye image-receiving sheet in which the dye image-receiving surface is not damaged by an adjacent sheet.